1. Field of the Invention
The present invention relates to a method of driving a display element that uses cholesteric liquid crystals, and particularly to a method of driving a display element by which a high-quality display with a multilevel halftone is realized.
2. Description of the Related Art
In recent years, electronic paper has been vigorously developed by companies and universities. Electronic paper can be applied to various portable devices including electronic books, sub-displays in mobile terminals, and display units in IC cards.
One effective way to realize electronic paper is to utilize cholesteric liquid crystals.
A cholesteric liquid crystal has excellent characteristics, including an ability to hold a display state semi-permanently (image memory characteristic) and to display images clearly in full color at a high contrast and at a high resolution. The cholesteric liquid crystal is also called a chiral nematic liquid crystal because the cholesteric liquid crystal is a nematic liquid crystal whose cholesteric phase is formed, and the cholesteric phase where molecules of the nematic liquid crystal are tied up in a helix is formed by adding a relatively large quantity (several tens of percent) of chiral addition (also called chiral material) to the nematic liquid crystal.
Hereinafter, the principles of the display and of the driving of cholesteric liquid crystals are explained.
A display using cholesteric liquid crystals is controlled in accordance with the oriented state of the molecules in the cholesteric liquid crystals. As shown in the graph of a reflectance in FIG. 1A, cholesteric liquid crystal shave a planar (P) state, where the incident light is reflected, and a focal conic (FC) state, where the incident light penetrates, and these states are stable even without an electric field. In the planar state, light having a wavelength corresponding to the helical pitch over the liquid crystal molecules is reflected. The wavelength λ that causes the maximum reflection is expressed by the equation below in which n is an average refraction index, and p is a helical pitch.λ=n·p 
In contrast, the reflection band Δλ increases as the refraction index anisotropy Δn increases.
Accordingly, by suitably selecting the average refraction index n and helical pitch p, it is possible to display a color having the wavelength λ in the planar state.
Also, by providing a light absorption layer separately from a liquid crystal layer, black can be displayed in the focal conic state.
Next, an example of driving cholesteric liquid crystals is explained.
When an intense electric field is applied to a cholesteric liquid crystal, the helical structure of the liquid crystal molecules are unwound completely and their state becomes homeotropic, with all the molecules oriented along the direction of the electric field. Next, when the electric field that has caused the homeotropic state suddenly becomes zero, the helical axis of the liquid crystal becomes perpendicular to the electrode, and the planar state is caused in which light is selectively reflected in accordance with the helical pitch. In contrast, when an electric field that is sufficiently weak so as not to unwind the helical structure is removed, or when an intense electric field is gradually removed after being applied, the helical axis of the liquid crystal becomes parallel to the electrode, and the focal conic state is caused in which the incident light penetrates. Also, when an intermediately intense electric field is applied and this electric field is removed suddenly, both the planar state and the focal conic state are caused and a display of halftones is possible.
By using this phenomenon, information is displayed.
The above voltage response characteristic can be described as follows by referring to FIG. 1A.
If the initial state is the planar state (P) (as indicated by the solid line), the driving band to the focal conic state (FC) is achieved when the pulse voltage increases to a certain range, and the driving band to the planar state is again achieved when the pulse voltage further increases.
If the initial state is the focal conic state (as indicated by the dashed line), the driving band to the planar state is gradually achieved as the pulse voltage increases.
When the voltages that are indicated in the zones of halftone zone A and halftone zone B are applied, a display of halftones is realized in which the above planar state and focal conic state are both caused.
Also, as shown in FIG. 1B, the cholesteric liquid crystals have the characteristic of a cumulative response, i.e., the cholesteric liquid crystals transit, with weak pulses being applied a plurality of times, from the planar state into the focal conic state, and from the focal conic state into the planar state.
If, for example, the initial state is the planar state, by successively applying a weak voltage pulse within the halftone zone A, the state gradually transits into the focal conic state in accordance with the number of times the pulse is applied, as shown in FIG. 1B. In contrast, as shown in FIG. 1B, when a weak voltage pulse within the halftone zone B is successively applied, the state gradually transits into the planar state in accordance with the number of times the pulse is applied regardless of which state the initial state was. This phenomenon indicates that a display at a desired halftone level can be realized by selecting the number of times to apply a pulse. Also, as shown enlarged in FIG. 1C, by gradually reducing the scatter reflections in the focal conic state, black can be displayed in a better state.
Next, by referring to FIG. 1D, electrodes that drive liquid crystals in a matrix liquid crystal display element are explained. Generally, a liquid crystal display element includes, as shown in FIG. 1D, a plurality of scanning electrodes 16 and a plurality of data electrodes 18 facing one another being arranged in such a manner that the scanning electrodes 16 cross the data electrodes 18. The spots at which the scanning electrodes 16 cross the data electrodes 18 serve as pixels. A scanning electrode driver 12 sequentially selects (common mode) one of the scanning electrodes 16, pulse voltage is applied to it, and pulse voltages corresponding to the display states of the respective pixels are applied to the data electrodes 18 by a data electrode driver 14 (segment mode); thereby, the liquid crystals of the corresponding pixels are driven. The difference between the voltage applied to the data electrodes 18 and the voltage applied to the scanning electrodes 16 is the voltage that is applied to liquid crystals of the pixels, and is the voltage that drives the liquid crystals shown in FIG. 1A.
Hereinafter, well-known prior art about methods of driving cholesteric liquid crystals with a multilevel halftone is described, each of which involves problems.
As disclosed in, for example, Patent Documents 1 and 2, there is a method called dynamic drive by which halftones are displayed by using amplitude, pulse width, or phase difference in the Selection stage in a driving wave that is divided into three stages: the Preparation stage, the Selection stage, and the Evolution stage. However, while this dynamic drive realizes quick operation, it causes large graininess in halftones. Also, generally, this dynamic drive requires a dedicated driver that can output voltages at several levels, and the cost increases because of the production of the driver and complexity of the control circuit.
Non Patent Document 1 discloses a dynamic drive that can be operated by an inexpensive and general purpose STN driver by improving the above dynamic drive. However, the problem of graininess is not solved by this dynamic drive.
Also, as a prior art method of driving halftones, there is a method disclosed in Patent Document 3 in which, by applying the second and third pulses immediately after applying the first pulse that makes liquid crystals into the homeotropic state, and a desired level of a halftone is displayed on the basis of the voltage difference between the second and third pulses. However, in this method, the probability of large graininess in halftones still remains, and also it is difficult to implement this method at a low cost because the driving voltage is high, which is problematic.
The above driving methods are methods in which the initial state does not matter and the halftone zone B is used; accordingly, even though it allows for quick operation, the graininess is large and the display quality is low, which is problematic.
Also, Non Patent Document 2 discloses another driving method that uses the halftone zone A. However, this method also has a problem.
In the method disclosed in Non Patent Document 2, short pulses are applied for using the cumulative response, which is peculiar to liquid crystals, and the liquid crystals are driven at a high speed of quasi moving-picture rate from the planar state to the focal conic state or from the focal conic state to the planar state.
However, in this method, the driving voltage can be as high as 50-70V because of the high quasi moving-picture rate, which causes a higher cost, and also causes a lower display quality because the “Two phase cumulative drive scheme” described in Non Patent document 2 uses the cumulative response in two directions, i.e., the cumulative response to the planar state and the cumulative response to the focal conic state (in other words, to the halftone zone A and the halftone zone B), by using the two stages “preparation phase” and “selection phase”.
As described above, a display with a multilevel halftone in electronic paper that uses the conventional cholesteric liquid crystals requires a driver IC that is specially designed to create driving waveforms at multiple levels, and the driving voltage can be as high as 40-60V, thus requiring the IC to have a high voltage endurance, which has lead to a higher cost. Also, the conventional techniques have a problem with graininess being large in halftones (low uniformity), and it is difficult to apply them to electronic paper that requires a high display quality.
Also, in the conventional techniques, halftone levels are controlled by switching voltage values of voltage pulses or pulse widths for each pixel selected. This requires the construction of a driver IC or a peripheral circuit that can arbitrarily switch voltage values or the pulse widths, which has caused a higher cost. Also, as is disclosed in Patent Document 1, there is a method of driving halftones, the method using a driver that has a smaller number of outputs. However, while this method allows for high-speed display updates, it also requires a driving voltage of up to 50-60V. Also, in this method, the driving margin of halftones is narrow, and graininess in halftones is large even when an element having a high uniformity in its cell gaps is used (for example with glass components), which has caused difficulty in realizing a high-quality display.    Patent Document 1:    Japanese Patent Application Publication No. 2001-228459    Patent Document 2:    Japanese Patent Application Publication No. 2003-228045    Patent Document 3:    Japanese Patent Application Publication No. 2000-2869    Non Patent Document 1:    A Novel Dynamic Drive Scheme for Reflective Cholesteric Displays, SID 02 DIGEST, pp 546-549, 2002. (Nam-SeokLee, Hyun-SooShin, et al.)    Non Patent Document 2:    Cumulative Drive Schemes for Bistable Reflective Cholesteric LCDs, SID 98 DIGESTS, pp 798-801, 1998 (Y.-M. Zhu, D.-K. Yang)